Solar modules serve for direct conversion of sunlight to electrical energy. As the most important constituent, they contain groups of solar cells which are connected electrically to one another in a suitable manner within the solar module and are embedded in different materials. These arrangements fulfil the following purposes, among others:                transparent, radiation- and weathering-resistant coverage of the solar cells,        protection of the brittle solar cell from mechanical influences,        protection of the solar cells and of the electrical connections from moisture:        protection from contact with the electrical components.        
Typically, solar modules consist of:                a transparent front cover, which is usually glass;        a transparent polymer material in which the solar cells are embedded;                    the most frequently used material is ethylene-vinyl acetate (EVA) in the form of films, which melts at relatively high temperature, is crosslinked by means of suitable additives and is thus extremely resistant to outside influences. According to the EVA type and the additives present therein, the crosslinking process requires a different amount of time. A distinction is drawn between so-called “standard cure” and “fast cure” types.                        a backside cover which may in principle be transparent or colored.                    Typically, polymer films or laminates of polymer films (e.g. of fluoropolymer and polyester films) are used. White nontransparent backside covers offer the advantage that light which falls between the solar cells is reflected from the upper side of the backside cover, which increases the efficiency of the solar modules.                        
In the production process of solar modules, front cover, EVA with solar cells and backside cover are bonded to one another in a vacuum laminator at temperatures of appox. 150° C. In order to achieve the required long lifetimes of solar modules of 20 to 40 years, it is necessary to ensure excellent durable adhesion of the front and backside covers to the EVA embedding material.
It would be desirable to have a white nontransparent polyester film with an EVA adhesion coating, which has excellent long-term stability to humidity and elevated temperature and is suitable for use as a backside cover in solar modules. The coating should have a good mechanical in order to withstand without damage the stresses which occur in the film production process, the winding and unwinding of the film and in the production of the solar modules. In order to meet the requirement for less expensive availability of solar power, the polyester film should also be producible in an economically viable manner; however, such white coated films are unknown to date.
EP-A-1 826 826 (whose United States equivalent is U.S. Patent Application Publication No. 2008/0050583A1) describes a polyester film provided with an EVA adhesion coating. The base film may also be colored white, black or another color; in the examples, however, only transparent films were used. The coating comprises a crosslinker and a polymer selected from the group of polyesters with a glass transition temperature of 20 to 100° C., acrylates with a glass transition temperature of 20 to 100° C., combinations of these polyesters and acrylates, and combinations of these polyesters and/or acrylates with polyvinyl alcohol having a degree of hydrolysis of 70 to 90 mol %. The crosslinkers used may, according to the publication, be urea or epoxy compounds, melamine or polymers containing oxazoline groups, the latter being described as particularly suitable. Owing to the known hydrolysis sensitivity of ester groups, polyester-based coatings, however, do not appear to be very suitable to ensure a good adhesion to EVA over a long period at high temperature and humidity. Polyvinyl alcohol tends to degrade and discolor even at elevated temperature in the production process of polyester films or under stress through light and heat in the solar module. The film claimed therefore hardly appears to be suitable for use as a durable backside cover in solar modules.
EP-A-1 908 587 (whose United States equivalent is U.S. Patent Application Publication No. 2009/0034235A1) claims a multilayer white film with good reflection properties for visible light, which is described as stable to discoloration under the influence of UV light. The good reflection properties for visible light are achieved by modification of a layer of the film with 31 to 60% by weight, and of a further layer with 0 to 30% by weight, of inert particles. According to the publication, barium sulfate is particularly suitable for this purpose. Another raw material used in addition to polyethylene terephthalate is polyethylene naphthalate. The film is said to be suitable primarily as a reflector in liquid crystal displays, but a possible use of the films for backside coverage in solar modules is also mentioned. Since, given such a high proportion of inert particles, tears in the film are barely avoidable in the production process, it is, however, doubtful whether such a film can still be produced with economically acceptable costs for solar applications too. Moreover, the publication does not mention how good adhesion to EVA at high temperature and humidity over a long period is ensured in the case of application in solar modules.